WASHINGTON (CNN) -- The Department of Defense may have paid more than $15 million into invalid civilian employee accounts over a six-year period, an investigation has concluded.

The review found that the Defense Department was paying into accounts with invalid Social Security numbers, that some employees' birth dates were below the legal age to work and that multiple employee wages were deposited into a single account.

The Defense Civilian Pay System (DCPS), which manages payroll for the department's 7.3 million civilian employees, was responsible for more than $148 billion in wages over the six-year period reviewed by the Department of Defense's Office of the Inspector General.

The $15.4 million deposited into invalid accounts is a very small percentage of the total wages. Still, the problems found by the review of deposits between January 2002 and April 2008 raise questions about oversight and vetting of employee information at the department.

The investigators found problems with invalid Social Security numbers that should have been relatively easy to spot, including three Social Security numbers with all zeroes in one section.

Social Security Administration guidelines state that "none of the three individual numbers of the SSN can be entirely comprised of zeroes," the report noted. In addition, the review found 98 Social Security numbers that were never issued by the Social Security Administration.

Inspector-general investigators also found $905,000 of wages paid to deceased employees because the DCPS "did not include procedures to identify deceased employees and recover improper payments." One employee's record reported date of death of March 1948, and the account still received $11,613 in 2002, the report revealed.

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Another 86 dead employees were found to have names in the Department of Defense payroll system that did not match their names in the Social Security register.

"These instances demonstrate the potential for fraud, because employees may submit the SSN of a deceased individual instead of their own," auditors warned.

The Defense Department also was paying employees whose ages in the records were under the legal employment age, including 1,921 accounts paid a total of $2.3 million for employees whose age, according to their records, were listed as being under 14 years.

Investigators also found multiple employee wages were being deposited into the same accounts. Its review turned up $11.9 million that was deposited into just 16 different accounts for 186 different employees.

The Defense Criminal Investigative Service is investigating the invalid accounts. The department overseeing payroll is implementing programs this year to spot and correct the problem accounts and prevent future occurrences of the error, according to the defense department.